Field
The following description relates to a technology for performing high-speed (or fast) signal search for respective signals of a Spread Spectrum and a Global Navigation Satellite System (GNSS), such as a Global Positioning System (GPS).
Discussion of Related Art
The GNSS collectively refers to a position measurement satellite system, such as a GPS, and the GNSS is used for measuring a Time of Arrival (TOA) of a radio wave (or frequency wave) being transmitted from a satellite antenna and received by a GPS receiver and, at the same, for calculating a relative position of a receiver by locating the position of a satellite corresponding to the point when the radio wave, which is received by the receiver, is transmitted from the satellite from the received signal. A signal being transmitted to the ground from all GNSS satellites corresponds to a Direct Sequence Spread Spectrum signal, which is used in a general wireless communication system. When driving (or starting) the receiver equipped in all types of communication and satellite navigation systems, one of the tasks that should be performed firsthand is to search for a signal and to acquire a code phase (i.e., accurate time) of a signal that is currently being received and an accurate frequency and required information (satellite position and error) of the signal that is being received.
When performing initial position acquisition of the GPS receiver, and when the GPS receiver has been in a power off state for a long period of time and then shifted back to a power on state in order to perform initial position measurement, this case is referred to as a Cold Start. And, in this case, the time that is consumed for performing a process of searching for a signal and acquiring the signal (a process of gaining an accurate code phase and frequency information and establishing synchronization with the received signal) is referred to as a Time To First Fix (TTFF). The cold start of a GPS receiver for searching for a Coarse Acquisition (C/A) signal that is being returned to an L1 frequency (1.575 GHz) of the GPS is realized in accordance with a plurality of process steps. And, most particularly, a step of searching for a code phase (corresponding to a time delay of a signal) hypothesis range and a Doppler frequency (occurring due to a relative speed between the satellite and the receiver) hypothesis range of a satellite signal requires a highest hardware complexity level. More specifically, since the GPS L1 frequency C/A code is configured of 1,023 code chips, the code phase hypothesis that should be searched (verified) corresponds to a total of 2,046 code phases in 0.5 chip units. Additionally, since the GPS satellite revolves around the Earth at a speed of approximately 3 Km/sec at an altitude of approximately 22,000 km, when observed from a ground receiver (or terrestrial receiver), the Doppler frequency of the GPS satellite signal may be generated within a range starting from −5 KHz to +5 KHz. Therefore, in case a (minimum) frequency search unit Δf corresponds to 500 Hz, a total of 21 Doppler frequency hypotheses should be verified at an interval of 500 Hz starting from −5 KHz to +5 KHz. Therefore, when considering all number of cases, the total number of search hypotheses is equal to 42,000. The GPS receiver is required to search for a C/A signal and to acquire a code phase and a Doppler frequency for the following reason. By establishing accurate synchronization with a GPS satellite signal that is being received at a current moment, the corresponding GPS satellite signal may be continuously tracked without being lost and an accurate signal wave time (i.e., distance) measurement value may be acquired, while, at the same time, satellite navigation information that is carried by (or included in) the GPS signal can be extracted.
The GPS receiver uses a correlator having a longer correlation length in order to detect an attenuated signal (or weak signal). In this case the Doppler frequency hypothesis should be searched in smaller units.
For example, in case the signal correlation length is equal to T=1 msec, the search for a Doppler frequency should be performed by searching for a minimum of 21 hypotheses in units of Δf<=1/(2T)=500 Hz, whereas, in case the signal correlation length is equal to T=100 msec, a minimum of 2,001 Doppler frequency hypotheses should be searched by using Δf=1/(2T)<=5 Hz. If a significantly long correlation length of T=1 sec is used, since the search process is performed in smaller units of Δf (=0.5 Hz) starting from −5 KHz to +5 KHz, a minimum total of 20,001 Doppler frequency hypotheses should be verified. At this point, the total number of hypotheses is equal to 20,001×2,046≈×107. Therefore, when considering the correlation length of T=1 sec, a receiver using only one correlator is required to perform signal search for a maximum time period of 4×107 seconds.
In order to perform Phase Coherent Correlation and Integration during a time period of T seconds for the search of one Doppler frequency and code phase, a process of gaining r[n]×r1[n] by performing correlation (i.e., multiplication) between received signal samples (r[n] for n=0, 1, 2, . . . , fsT−1, wherein fs corresponds to a sampling frequency), which are received during a time period of T seconds, and receiver replica signal r1(t) samples (r1[n] for n=0, 1, 2, . . . , fsT−1) having the same Doppler frequency and code phase, and comparing an integration result value Z(=|Σn=0fsT−1r[n]r1[n]|2) corresponding to an integration of the correlation (multiplication) results between the samples with a direction threshold value (γ). At this point, if the Doppler frequency or code phase of r1(t) is not the same as that of the signal being received, the value of Z has a value equal to or smaller than the threshold value. Therefore, the receiver searches for a signal by performing the correlation and integration process with respect to the combination of all Doppler frequencies and all code phases.
As described above, generally, as the value of T increases, and as the sampling frequency (fs) respective to the signal being received becomes greater, the integrated number of samples (=fsT) is increased, and the computational load required for the hypothesis search is also increased. Therefore, as the value of T increases, since the number of hypotheses that are required to be verified (or tested) increases, and since the computational load also increases at the same time, computational resources that are consumed in order to detect an attenuated signal increases at an exponential rate. As described above, despite the complexity in the process of performing signal search by verifying numerous hypotheses and acquiring the detected signal, in order to quickly locate the position of the receiver, since the verification and acquisition process should be performed at the same time as the receiver is turned on, a significant amount of hardware resource is instantly concentrated in performing the signal acquisition process, and, after completing the signal acquisition process, the hardware resource is no longer used. Accordingly, this process is very inefficient.
Diverse technologies have been researched and developed in order to accelerate the performance of such signal detection, and, among such technologies, one of the simplest technologies corresponds to a method of using multiple parallel correlators. For example, each of 42,000 parallel correlators simultaneously verifies the hypotheses of the code phase and Doppler frequency combination within a time period of 1 msec, signal acquisition may be quickly completed. However, since a considerably large number of correlators is used for only one time, in the aspect of using hardware resources, this method is highly inefficient. Another method corresponds to a method of performing a very fast correlation by multiplying the received signal by a code signal (PRN code signal), which is generated from the inside of the receiver, in a frequency domain by using Fast Fourier Transform (FFT) (FFT-based technology). And, in order to do so, the received signal r(t) and the signal generated from the inside of the receiver r1(t) should be processed with Fourier Transform in all frequency domains, and, after performing multiplication in the frequency domain, the multiplication result is required to be processed with Inverse Fourier Transform back to the time domain. Therefore, the correlator is not required in this process, yet the computational load becomes extremely high. The above-described FFT-based signal search technology having the high computational load may be realized by using a DSP chip, which is equipped with high speed. And, since the DSP chip may be used for purposes other than the signal search, the hardware inefficiency may be enhanced. However, due to the usage of the DSP chip, the fabrication cost may become another problem.
In the above-described FFT-type signal search technologies, the conventional technology that can reduce the computational load includes an averaging correlator (AC) technology and a shifting replica (SR) technology. In the AC technology, in a case when two or more sampling frequencies are given per chip (i.e., fs=qcRsRc, qcRs>=2, wherein Rc is a chip rate, fs is a sampling rate, qc is the number of compressed samples per chip and Rs is a sample compression rate), an average of qcRs number of consecutive samples is calculated, so as to generate a received signal, which is indicated for each chip, and a signal sample is generated for each chip within the receiver, thereby carrying out the correlation. The AC technology is advantageous in that correlation may be performed by using a number of signal samples that is reduced to qcRs time the number of signal sampled used in the conventional method. However, since a highest correlation result is obtained when consecutive samples having the average value belong to the same chip, a sample starting point having the highest integration result is located by performing qcRs number of correlation/integration sessions based upon qcRs number of different sample starting points. Accordingly, the AC technology performs a process step of generating an average signal having a total of qcRs different sample offsets and finding a maximum value from qcRs number of correlation results. Generally, when FFT-based correlation is performed from N number of complex signal samples, since N log2N number of multiplications are required for the N number of sample multiplications and IFFT, a total of N(log2N+1) number of complex multiplications are required to be performed. Therefore, when fs=qcRsRc, since N=qcRsRcT, in case of the conventional technology, qcRsRcT(log2qcRsRcT+1) number of complex multiplications are required to be performed. However, in case of the AC technology, qcRs(RcTlog2RcT+RcT) number of complex multiplications are performed and used.
Another technology corresponds to the SR technology, wherein T=RTT1 (herein, T1=1 ms), and wherein a received signal is sampled by using a frequency of fs(=qcRsRc), so as to create a two-dimensional (2D) matrix U having a size of [qcRsRcT1×RT], so that the computational load can be reduced by using a 2D FFT respective to U. When using the SR technology, 2qcRsRcT1RT[log2(2qcRsRcT1)+log2(RT)+1] number of complex multiplications are performed and used with respect to the received signal, and since 42qcRsRcT1RT[log2(2qcRsRcT1)+1] number of multiplications are performed and used in the frequency domain, the computational load may be reduced to 10 to 100 times that of the AC technology.
However, in the conventional technologies, such as the AC technology and the SR technology, Phase Coherent Correlation and Integration cannot be performed during a time period (T>>Tb) that is longer than the time length (Tb) of data carried by the signal. More specifically, during an initialization process of the receiver, wherein the received data value (+1 or −1) is unknown, when the Phase Coherent Correlation result is being integrated, since the phase of the correlation result may be changed by 180 degrees)(180° due to a change in the data value, the size of the integrated result may be reduced instead of being increased, and this may eventually lead to a failure in the signal detection. Therefore, in the conventional technology, a Non-coherent integration procedure is performed, wherein the phase coherent correlation length is limited to Tco(=5 ms or 10 ms), which is shorter than Tb, and wherein, after acquiring an absolute value of the phase coherent correlation and integration result during the time period of Tco seconds, the acquired absolute value is added to an absolute value of the phase coherent correlation and integration result acquired during the next Tco seconds, and wherein this process is repeatedly performed. This Non-coherent integration procedure is repeated for Nnc number of times, and, in case of a severely attenuated signal, since the amount of energy of the signals being accumulated by using the phase coherent method is small, and since the Signal-to-Noise Ratio (SNR) is reduced during the process of acquiring the absolute values, the Nnc is generally set up to have a high value (i.e., TcoNnc>>T), so that the SNR of the final non-coherent integration result can have a sufficiently high value. As a result, in the above-described non-coherent technology, since TcoNnc>>T, a larger amount of time is consumed in order to test one search hypothesis.